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| United States Patent |
5,349,240
|
|
Narita
, et al.
|
September 20, 1994
|
Semiconductor device package having a sealing silicone gel with
spherical fillers
Abstract
A semiconductor element mounted on a board is sealed with a sealing resin,
The semiconductor element is electrically bonded to the board through a
soldering bump. The sealing resin includes a silicone gel as a base resin
and a filler filled into a silicone gel. A diameter of the filler is equal
to or shorter than the distance between the semiconductor element and the
board, and a shape of the filler is spherical to improve a thermal
conductivity of the sealing resin. The relationship between a coefficient
.alpha. of a linear expansion and a complex modulus of elasticity G* of
the sealing resin is defined as follows to reduce a force pushing up the
semiconductor element:
.alpha..ltoreq.0.033(G*-451).sup.-0.56.
| Inventors:
|
Narita; Ryoichi (Obu, JP);
Fukuda; Yutaka (Kariya, JP)
|
| Assignee:
|
Nippondenso Co., Ltd. (Kariya, JP)
|
| Appl. No.:
|
965919 |
| Filed:
|
October 26, 1992 |
Foreign Application Priority Data
| Current U.S. Class: |
257/791; 257/787; 257/788; 257/789; 257/795; 257/E23.087; 257/E23.14 |
| Intern'l Class: |
H01L 023/28; H01L 023/02 |
| Field of Search: |
257/787,788,789,791,795
|
References Cited
U.S. Patent Documents
| 3492157 | Jan., 1970 | Ito et al.
| |
| 3751724 | Aug., 1973 | McGrath.
| |
| 4282136 | Aug., 1981 | Hunt et al.
| |
| 4337182 | Jun., 1982 | Needham.
| |
| 4812897 | Mar., 1989 | Narita et al.
| |
| 4965660 | Oct., 1990 | Ogihara et al.
| |
| 5015675 | May., 1991 | Walles et al.
| |
| 5225499 | Jul., 1993 | Kokaka et al. | 257/788.
|
| Foreign Patent Documents |
| 57-212225 | Dec., 1982 | JP.
| |
| 59-149037 | Aug., 1984 | JP.
| |
| 61-76587 | Apr., 1986 | JP.
| |
| 62-96538 | May., 1987 | JP.
| |
| 62-240313 | Oct., 1987 | JP.
| |
| 63-15449 | Jan., 1988 | JP.
| |
| 63-183915 | Jul., 1988 | JP.
| |
| 2-324872 | Nov., 1990 | JP.
| |
Primary Examiner: Crane; Sara W.
Assistant Examiner: Whitehead, Jr.; Carl
Attorney, Agent or Firm: Cushman, Darby & Cushman
Claims
What is claimed is:
1. A resin-sealed semiconductor device comprising:
a board having a conductor pattern formed thereon;
a semiconductor element including at least one soldering bump having a
predetermined height facing said board and electrically bonded to said
conductor pattern through said soldering bump; and
a sealing resin entering a space between said semiconductor element and
said board and sealing said semiconductor element;
wherein said sealing resin includes
a silicone gel as a base resin; and
a spherical filler filled into said silicone gel, a diameter of said filler
being at most a distance between said semiconductor element and said
board, and volume percentage of said filler in said silicone gel being
adjusted in such a way that a relationship between a coefficient .alpha.
of linear expansion of said sealing resin and complex modulus of
elasticity G* (dyn/cm.sup.2, at 1H.sub.z, 30.degree. C.) of said sealing
resin meets retirement of following formula:
.alpha..ltoreq.0. 033(G*-451).sup.-0.56.
2. A resin-sealed semiconductor device according to claim 1, wherein said
semiconductor device is a flip-chip IC.
3. A resin-sealed semiconductor device according to claim 1, wherein said
silicone gel is of an additional reaction type.
4. A resin-sealed semiconductor device according to claim 1, wherein
thermal conductivity of said filler is higher than thermal conductivity of
said silicone gel.
5. A resin-sealed semiconductor device according to claim 4, wherein said
filler is made of at lease one of alumina, aluminum nitride and crystal
silica.
6. A resin-sealed semiconductor device according to claim 5, wherein said
filler is made of alumina.
7. A resin-sealed semiconductor device according to claim 1, wherein said
device is used under a condition of a temperature change width up to
200.degree. C.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a resin-sealed semiconductor device in
which a semiconductor element such as a flip-chip mounted on a hybrid
substrate is sealed with a resin.
2. Description of Related Art
A conventional semiconductor chip is sealed with silicone gel or the like
to insulate electrically and to protect the chip from environmental
hazards such as water. A hybrid IC (Integrated Circuit) including, a
flip-chip, for example, is electrically and mechanically connected with a
conductor pattern formed on a thick layer circuit board through soldering
bumps formed on a main surface of the flip-chip. The board is packed or
sealed with silicone gel.
The silicone gel entering a space between the flip-chip and the board is
thermally expanded and pushes up the flip-chip. U.S. Pat. No. 4,812,897
(corresponding to Japanese unexamined patent publication No. Showa
62-149157) suggests to soften the silicone gel in order to reduce the
force pushing up the chip and to prevent breakage of the soldering bumps.
Therefore, a silicone gel which has a low cross linking density has needed
to be used.
A sealing resin needs to be filled with a filler which has a high thermal
conductivity in order to reduce a generation of heat by the chip. However,
since the silicone gel needs to be hardened in this case, it finally
breaks the soldering bumps.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a
resin-sealed semiconductor device sealed with a silicone gel having a good
thermal conductivity and a low stress for a semiconductor element at the
same time.
To accomplish the foregoing and other objects, the resin-sealed
semiconductor device of the present invention includes a board, a
semiconductor element and a sealing resin to seal the semiconductor
element. The board has a conductor pattern formed thereon. The
semiconductor element faces the board and is electrically bonded to the
conductor pattern through a soldering bump having a predetermined height.
The sealing resin has a silicone gel as a base resin and a spherical
filler filled into the silicone gel. The diameter of the filler is equal
to or shorter than the distance between the semiconductor element and the
board. The relationship between a coefficient .alpha. of linear expansion
and a complex modulus of elasticity G* of the sealing resin is defined as
follows: .alpha..ltoreq.0.033 (G*-451).sup.-0.56.
Since the silicone gel is filled with the spherical fillers, thermal
conductivity of the sealing resin is improved. Moreover, since the
relationship between .alpha. and G* is defined by the above-mentioned
formula, a force pushing up the semiconductor element is reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the present invention that are believed to be novel are set
forth with particularity in the appended claims. The invention, together
with the objects and advantages thereof, may best be understood by
reference to the following description of the presently preferred
embodiments together with the accompanying drawings in which:
FIG. 1 is a cross-sectional view of an embodiment of a resin-sealed
semiconductor device according to the present invention;
FIGS. 2, 3 and 4 are schematical side views of the semiconductor device
shown in FIG. 1, for explaining shearing strain and tensile strain
produced by thermal stress in a portion of an IC chip and bumps;
FIG. 5 shows the relationship between the complex modulus of elasticity and
a coefficient of linear expansion;
FIG. 6 shows the relationship between the volume percentage of alumina and
the complex modulus of elasticity;
FIG. 7 shows the relationship between the volume percentage of alumina and
the coefficient of linear expansion;
FIG. 8 shows the relationship between the volume percentage of alumina and
a thermal conductivity;
FIG. 9 shows the relationship between the complex modulus of elasticity and
the coefficient linear expansion; and
FIG. 10 shows the relationship between kinds of sealing resin and a steady
head resistance.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The preferred embodiment of the present invention is now described with
reference to the drawings.
FIG. 1 shows a general structure of an automotive hybrid IC according to
the present embodiment.
A heat sink 1 is made of metal. A thick layer circuit board 2 made of
alumina is installed on the heat sink 1. Predetermined conductor patterns
3 are formed on the board 2. A flip-chip IC 4 made of silicon is soldered
to the conductor patterns 3. Namely, soldering bumps 5 are formed on a
main surface of the flip-chip IC 4, and the flip-chip IC 4 faces the board
2 through the soldering bumps 5. A power transistor 6 is also formed on
the board 2. The power transistor 6 is electrically connected with the
conductor pattern 3 through a wire 7. A case 8 circumferentially bonds to
an upper surface of the heat sink 1. A cap 9 closes an upper opening of
the case 8. The case 8 and the cap 9 are made of metal or resin. A sealing
resin 10 fills in the case 8 to seal the board 2, the flip-chip IC 4 and
the power transistor 6.
This structure of the hybrid IC generally suffers repeated temperature
cycles due to ambient temperature changes and the self generation of heat,
and the strain caused by the repeated thermal stresses affects the
flip-chip IC 4.
The thermal strain is now explained in detail with reference to FIGS. 2-4.
The thermal strain comprises shear strain .delta.s shown in FIG. 3 and
tensile strain .delta.E shown in FIG. 4. A thermal expansion coefficient
of silicon is 4 ppm/.degree.C. A thermal expansion coefficient of alumina
is 7 ppm/.degree.C. The shear strain .delta.s is produced by a difference
between the thermal expansion coefficients of the flip-chip IC 4 and the
board 2. The tensile strain .delta.E is produced by the thermal expansion
of the sealing resin 10 inserted in a gap H having the height of the
soldering bumps 5. The tensile strain .delta.E pushes up the flip-chip IC
4. The shear strain .delta.s and the tensile strain .delta.E increase with
any increase of the chip size of the flip-chip IC.
The tensile strain .delta.E affects breakage of the soldering bumps 5 more
than the shear strain .delta.s in a sealing structure in the prior art.
Therefore, the first mode of breakage of the soldering bumps occurs due to
pushing-up the flip-chip IC 4 by thermal expansion of the sealing resin 10
which is directly below the flip-chip IC 4. It is desirable that the force
from the sealing resin 10 pushing up the flip-chip IC 4 is reduced in such
a way that breakage due to the shear strain .delta.s appears prior to
breakage due to the tensile strain .delta.E. In this case, enlargement of
the flip-chip IC 4 and extension of the thermal fatigue life of the
soldering bumps 5 due to repeated temperature cycles can be attained, and
as a result, higher and more varied functions can be attained in a single
chip. The reliability of the whole device involving the sealing structure
can also be increased.
The force Fg pushing up the flip-chip IC 4 shown in FIG. 4 is represented
by the following formula (1), which is derived from a theoretical
visco-elasticity analysis in which flow, deformation and the like of
various elastic viscous materials are analyzed:
##EQU1##
wherein G* denotes the complex modulus of elasticity of the sealing resin
10; S the area of the flip-chip IC 4; .DELTA.T the width of temperature
change around the flip-chip IC 4; .alpha. the coefficient of linear
expansion of the sealing resin 10; H the distance between the flip-chip IC
4 and the board 2; and a, b, c, d and e are constants.
This formula shows that the pushing up force Fg can be calculated if G*
representing the hardness of the sealing resin 10, i.e., the complex
modulus of elasticity G* and the coefficient .alpha. of linear expansion
are determined.
The pushing up force Fg must be equal to or larger than a pushing up force
F.sub.IC as represented by the following formula (2), wherein the
flip-chip IC 4 must be able to withstand F.sub.IC without breakage.
F.sub.IC .gtoreq.Fg=a(G*-b).sup.c .multidot.S.sup.d
.multidot..DELTA.T.multidot..alpha..multidot.H.sup.e (2)
The relationship between the complex modulus of elasticity G* and the
coefficient .alpha. of linear expansion is easy to understand by
transforming the formula (2) into the following formula (3).
.alpha..ltoreq.F.sub.IC .multidot. (a(G*-b).sup.c .multidot.S.sup.d
.DELTA.T.multidot.H.sup.e).sup.-1 (3)
Here, if the sealing resin 10 is not used and air is present between the
flip-chip IC 4 and the board 2, the maximum size of the flip-chip IC 4
which may be used, considering breakage thereof due to the tensile strain
.delta.E, is usually assumed to be 7 to 9 mm.quadrature.. Thus, the size
of the flip-chip IC 4 is assumed to be 7 mm.quadrature. as the worst case.
Moreover, although the force F.sub.IC is determined in accordance with the
number, shape, material, position or the like of the soldering bumps 5,
these factors should usually be designed in a limited range and thus
should be considered as a constant parameter. The range of temperature
change, .DELTA.T max, of an automotive IC may be as high as 200.degree. C.
The formula (3) can be reduced to the following formula (4), when a
hypotheszed factor, such as a boundary condition, is treated as a
correction constant. The correction constant was experimentally sought by
experimenting with some appropriate IC chips.
.alpha..ltoreq.0.033(G*-451).sup.- 0.56 (4)
wherein the unit of the complex modulus of elasticity G* is dyn/cm.sup.2,
at 1H.sub.z, 30.degree. C.
Therefore, the relationship between G* and .alpha. needs to meet the
requirement of the formula (4) to reduce sufficiently the pushing up force
Fg. FIG. 5 shows the relationship.
It is desirable to fill the sealing resin 10 with fillers having a high
thermal conductivity in order to improve the thermal conductivity of the
sealing resin 10. Silica, alumina, silicon carbide, silicon nitride,
aluminum nitride, magnesia and diamond are respectively suitable for such
a filler. The base resin of the sealing resin 10 according to the present
embodiment is an additional reaction-type silicone gel. Spherical alumina
fillers are filled into the silicone gel. Alumina does not have a
hydrolysis characteristic, keeps a high reliability for a long time, and
has a low cost. A maximum diameter of the spherical alumina filler is
equal to or shorter than the distance H between the flip-chip IC 4 and the
board 2.
FIGS. 6-9 show characteristics of crushed alumina fillers and spherical
alumina fillers, wherein both alumina fillers are filled into an
additional reaction type silicone gel which has the complex modulus of
elasticity G* of 1200 dyn/cm.sup.2. A maximum diameter of the crushed
alumina fillers is 20 .mu.m, and average diameter of them is 5 .mu.m. A
maximum diameter of the spherical alumina fillers is 50 .mu.m, and average
diameter of them is 10 .mu.m. A ratio of the major axis to the minor axis
of the spherical alumina filler is 1:1. White circles are data points on a
graph for crushed alumina fillers and black circles are data points on a
graph for spherical alumina fillers in FIGS. 6-9.
As a quantity, i.e., volume percentage of the crushed alumina fillers is
raised to improve the thermal conductivity of the silicone gel, as shown
in FIG. 6, the complex modulus of elasticity G* of the silicone gel is
rapidly raised. Therefore, as shown in FIG. 9, when the thermal
conductivity is higher than 0.9.times.10.sup.-3 cal/cm.sec. .degree.C.,
the silicone gel does not meet the requirement of the above-mentioned
formula (4). The data point D1 marks a point where the crushed alumina
filler has a thermal conductivity of 0.9.times.10.sup.-3 cal/cm.sec.
.degree.C. and line L shows .alpha.=0.033(G* 451).sup.-0.56. Above line L
is an undesirable region and below it is a desirable region.
On the other hand, as volume percentage of the spherical alumina fillers is
raised, the complex modulus of elasticity G* of the silicone gel is
relatively slowly raised. Therefore, as shown in FIG. 9, the silicone gel
meets the requirement of the formula (4) until its thermal conductivity
exceeds sufficiently high thermal conductivity (=higher than
2.5.times.10.sup.-3 cal/cm.sec. .degree.C.) the data point D2 shows the
spherical alumina filler having the thermal conductivity of
2.5.times.10.sup.-3 cal/cm.sec. .degree.C. The reason is that, as shown in
FIGS. 7 and 8, although the coefficient .alpha. of linear expansion and
the thermal conductivity are affected only by the volume percentage of the
alumina fillers without an influence of shape of them as shown in FIG. 6,
the complex modulus of elasticity G* is fairly affected by the volume
percentage and the shape of the alumina fillers.
It is noted that, if the maximum diameter of the filler is longer than the
distance H between the flip-chip IC 4 and the board 2 it is impossible to
sufficiently improve the thermal conductivity.
FIG. 10 shows a steady heat resistance of the power transistor 6, shown in
FIG. 1, which is sealed with the sealing resin 10 having the thermal
conductivity of approximately 2.5.times.10.sup.-3 cal/cm.sec. .degree.C.
The heat resistance of the sealing resin 10 according to the present
embodiment, i.e., the silicone gel including the spherical alumina fillers
is lower than that of the silicone gel not including such fillers.
As mentioned above, according to the present embodiment, since the silicone
gel as the sealing resin 10 is filled with the spherical alumina fillers,
the thermal conductivity of the sealing resin 10 is improved; therefore,
the heat generated from the power transistor 6 is promptly released.
Moreover, since the spherical alumina fillers are filled into the silicone
gel in such a way that the relationship between G* and .alpha. of the
sealing resin 10 meets the requirement of the formula (4), the pushing up
force Fg due to the tensile strain .delta.E is reduced. Consequently, the
sealing resin 10 has a good thermal conductivity and a low stress for the
flip-chip IC 4 at the same time.
The present invention has been described with reference to the
above-mentioned embodiment, but the present invention is not limited to
this embodiment and can be modified without departing from the spirit or
concept of the present invention.
For example, spherical fillers having high thermal conductivity, such as
aluminum nitride, or crystal silica are also suitable for the
above-mentioned sealing resin 10.
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